In typical control valves, a valve cage may provide guidance for a valve plug as the valve plug moves from a closed position in which the valve plug sealingly engages a valve seat to an open position in which the valve plug is disposed away from the valve seat. When the valve is in the open position, fluid flows from a valve inlet, passes through a passage between the valve seat and the valve plug, passes through the valve cage, and exits through a valve outlet. Valve cages can be comprised of an assembly of a plurality of disks that are arranged in a vertical stack. The disks generally have a plurality of inlet passages, at least one plenum section, and a plurality of outlet passages. In such disks, fluid enters the inlet passages, passes through the plenum sections of adjacent disks, and exits through the outlet passages. In such stacked disk valve cages, fluid exiting the outlet passages can create high noise levels that can be capable of permanently damaging the hearing of persons working close to the control valve.
Stacked disks, such as those described above, may also be used in a vent diffuser to reduce the pressure of fluid, such as steam, exiting the diffuser to the atmosphere. As discussed above, in such stacked disk vent diffusers, fluid exiting the outlet passages can create high noise levels.
To reduce the overall noise level of the fluid exiting the valve cage or vent diffuser, a common practice is to break up flow jets into the smallest possible independent jets. Breaking up the flow jets in this manner shifts the frequency of the noise generated by fluid exiting the valve cage, which can be shifted to a range that is near to or above the range that can be sensed by the human ear. The smaller the size of the outlet passage, the greater the apparent attenuation due to the shift of the noise. However, the smaller the size of each outlet passage, the greater the number of outlet passages that are necessary to handle the fluid passing through the valve cage. Consequently, increasing the number of outlet passages increases manufacturing costs.
In addition to breaking up the flow jets, the outlet passages can also be made of two or more different widths, which allows the outlet passages to cooperate to shift the noise peak frequencies of the fluid exiting the passages. However, even using identical stacked disks having outlet passage of different widths, the stacked disks still form a vertical pattern of outlet passages in close proximity that all exhaust in the same direction when stacked together. In this vertical pattern, the exiting jets can combine in a vertical manner since they are all aligned and exhausting in the same direction causing flow/pressure tones that lead to higher noise levels detectable in the human audio range.